Push-pull wide band amplifier



1, 1953 R. J. ROCKWELL PUSH-PULL WIDE BAND AMPLIFIER 3 Sheets-Sheet 1 Filed 00";- 4. 1949' \llual. i

INVEN TOR.

.RO/VALD J. ROCKWELL Arron/5% 1, 1953 R. J. ROCKWELL PUSH-PULL WIDE BAND AMPLIFIER 3 sheets-sheet 2 Filed 001:. 4, 1949 1 ATTORNEiS' 1, 1953 R. J. ROCKWELL 2,648,727

PUSH-PULL WIDE BAND AMPLIFIER Fiied Oct. 4, 1949 s Sheets-Sheet 5 Ei 9 'Z.

INVENTOR.

RONALD J. ROG/(WELL A TTOR/VE Y .1six;-illustrative embodiments of amplifier in ac Patented Aug. 11, 1953 UNITED @STATES TENT QiFFIiCE PUSH -PULLWIDE BAND AMPLIFIER .'-Ronald J. ;-Rockwell, Cincinnati, 0hio,zassi gnor tosCrosley Broadcasting Corporation; Cincin- .:nati,-0hi0,-,a corporation of Ohio Application 'Gctober 4, 1949, Serial No. 119,401

,The present :inventionrrelates :to amplifiers and specificallyrto arhigh-fidelity audio amplifier of the ypush-pullbutput type.

A-primaryobjectiofathapresent invention is "to provide a-;push-pull or balanced output'ampli- :fier which is -,characterized'by' a very high degree of freedom :from;u'ndesired:harmonics.

"Anothertobject of :the present invention is to -z-provide--a-two-.tube amplifier having two output iterminalsysaid amplifier being so arranged that the characteristics of. both tubes are manifested {at eachvterminal andthat the signals appearing at each terminal are "the resultant of the continuous operation of both tubes.

imade to thefollowing description of the accom- TpanYing drawings, in which there are described -'cordance with the invention, all ofthese embodiments having generic. inventive attributes in common.

In the drawings: Fig. 1 is a circuit diagram of a'single-ended 1 output signal translating circuit or amplifier, and

.it represents an improvement upon the basic inavention illustrated in Fig. '7 of my US. Patent 2,446,025 issued July 27, 1948, and applied for May 10, "1946;

'Fig. 2Iis-a' circuit schematic of a push-pull or balanced type of amplifier which incorporates the novel. features of the amplifier shown in Fig. 3 is a circuit schematic of a modified form 0f the invention, in which single iron core chokes -are substituted for the'pairs of chokes shown in the-cathode and plate circuits of Fig. 2, in which the anodecircuit choke is not inductively coupled '-.to the cathode circuit choke;

Fig. 4 is a circuit schematic of another embodiment of the invention, which differs from the Fig. 3 circuit in that the driving tubes are shown in detail, and -further in that the output capacitors 35 and 37 are shown, the novel arrangements for increasing; the efifeotive anode potentials of the-driver tubes being omitted;

Fig. 5 disclosesanother embodiment resembling the Fig. 3 circuit and dififering therefrom in the respect that a transformer secondary d2 is provided for a signal output circuit;

Fig. 6 illustrates an embodiment which is similar to Fig. 2,1anol departs therefrom primarily in the following respects: 1) *in the showing of a push-pull transiormer input; '(2) in the pro- 5 :Glaims. (Cl. 179171) 2 vision of a transformer-type.output cirouit being illustrated; and the Fig. T embodiment is :a schematic-;diagram-- of a form of "the invention'which closelyfiresembles that shown in Fig. "2, inductance -meansmgain being provided .for cooperating with I the capacitance means for coupling the'cathodesto'the output terminals, and' the single eplatewchoke and single cathode tchokebeingwwoundon the same core.

In accordance withith'e: invention-:thereis provided a push-pull amplifiercomprisingaa pa-irof vacuum tubes having rcathode circuitsvand anode circuits anda'pair of outputterminalsea cathode circuit of one of said tubes andan anode cir'cuit of another of saidtubesbeinghcoupled to each: of

said terminals. I

Referring now specificallyto' Fig.1; this -"circuit resembles inf'manytrespects =th'at" illustrated in Fig.7 of my"said Patent2,446,025. .'I'he-.Fig. 1 embodiment comprises a :pair of driv'en :tubes 5 and 2 for providingra push-t-pullinput signal to the grids ofapain of amplifier=tubes-al- 3 and l4.

These amplifier tubes :are shown as triod'es, al-

though it will "be understood wthat o-triodes,

pentodes, and tetrodes are equivalents;' The driver tubes are coupled-Ito the amplifier tubesby the resistance-capacitance couplingsnetworks,

each consisting of azpl'atefload resistor,-'-a 'grid resistor, and a 'capa'citor. x Specifically; the network between driver "tube '1 and amplifier tube is includes a 'plate load resi'stor l9, '2. coupling capacitor 20, "and a grid resistor 2i. Likewise, the network between driver tube 2 and amplifier tube it comprises a'plateiloacl 'resistor'125, acoupling capacitor 26, and a grid resistor 21." The junction of the grid?resistorsis connected=to a source of negative biasing "potential indicat'edby the symbol 'C; "Theiplatesoftubeswland-i1tare connected to a-source ofspace' current indicated by the symbol -|'B. The plate iofwtu'b'e --l =3' is connected to +B throughplate choke32. -The plate of tube '2 'is connected to +Bnthrough a series circuit comprising ilo'ad res'istor"25 -*and plate choke 32.

In the Fig. 1 embodiment the output-signal is available between terminal H "and= ground,-: the space current source "being included *in series in this output circuit. The negative portion of the output signal wave is :produced by 'the plat'e" output of tube l3,"andit-appearsacrossiplate choke 32, connected between'the :anode: of tube Jar-and a source 'of'space current indicated "by the areference numeral tB. The junction of the anode-of tube lit-and the ChOKQlBZJCOIJSt'ItUtBS anwoutput terminal 4!. The positive portion of the output signal wave is taken from the cathode circuit of tube I4, which cathode circuit includes a choke 34, the high potential side of which is coupled by a capacitor 39 to the output terminal 41.

The signal translating circuit illustrated in Fig. 1 departs from that shown in my patent in that the chokes 32 and 34 are wound on the same iron core, and so poled that the direct current component flowing in choke 32 is of opposite polarity to that flowing in choke 34. The two direct current components tend to cancel each other out, therefore to minimize the resultant D. C. component which magnetizes the core. As a consequence, low frequency fidelity is improved and the problem of incremental inductance is satisfactorily solved. The Fig. 7 showing of my aforesaid patent requires that a large capacitor 39 be employed for coupling the cathode circuit of tube M to the output terminal 4|. A 30 microfarad capacitor is there used. The size of this capacitoris radically reduced because of the supplemental coupling provided by winding choke 32 on the same core as choke 32. Accordingly, the chokes 34 and 32 are illustrated as being wound on the same iron core. The coupling between the chokes 32 and 34 is loose, and it improves the low frequency response. Even a small value of capacitance for the element 39 assures an excellent high frequency response. The combination of capacitive and inductive coupling of the cathode circuit of tube M to the output terminal 4| and the direct coupling of the anode circuit of tube l3 to the output terminal 4| assure a very flat and faithful response throughout a wire range of audio frequencies.

The Fig. 1 circuit relies primarily on capacitor 39 for coupling the cathode circuit of tube Hi to terminal 4|, and the chokes 32 and 34 are loosely coupled and mounted on the same core for the following purposes:

First, to supplement the coupling provided by capacitor 39, permitting a capacitor of relatively small size to be used; and

Second, to improve low-frequency-response by minimizing the D. 0. component of the ma netizing current for the core.

The provision of capacitor 39 renders the circuit of particular utility for heavy power requirements, which requirements are not satisfied by m the use of bifilar windings for coupling purposes. One of the advantages of the present invention is the fact that bifilar windings are dispensed with.

The Fig. l circuit includes another feature I shown in my said Patent 2,446,025, in that, during the negative grid excursion of driver tube 2, its positive plate excursion is augmented by the positive plate excursion at terminal 4 I, thus producing a positive bias excursion at the grid of tube [4 which is in excess of the positive cathode excursion of tube [4. Tube i4 is highly degenerative and requires a positive grid excursion with respect to ground which is at least equal to its cathode excursion plus its grid bias. The

network M, 25, 2'6, 21 in effect regeneratively couples the output reactor 32 to the input of tube I4. As stated, the effective plate potenial of driver tube 2 is increased in a positive direction as the cathode of tube M goes positive. Conversely, when the plate circuit of tube I3 is developing its negative output wave portion, capacitor 39, being connected between terminal 4| and the cathode of tube It, applies a negative signal to the cathode of tube [4. This feature a the advantage that tube M is strongly driven.

The principles of the Fig. l embodiment are utilized in the Fig. 2 embodiment. Only the anodes of the driver tubes need be shown in the illustration of the Fig. l embodiment. The driver tubes are omitted in the other embodiment s, but it will be understood that suitable push-pull driving circuits are employed in conjunction with each.

Like reference numerals are used throughout the specification and drawings to designate like elements, and therefore it will be understood that in the description of the Fig. 2 embodiment, elements bearing the same reference numerals as in Fig. 1 need not be further described.

In Fig. 2 individual chokes 3!, 34 are provided for the cathode circuits of tubes 13, is. And ndividual plate chokes 32, 35 are also arranged in series across the plates of the tubes, the junction of the plate chokes being connected to a source of space current +B- to shunt-feed tubes l3, M. The balanced arrangement of Fig. 2 includes two output terminals 4|, 40. Terminal ii is directly connected to the anode of tube 13, capacitively coupled to the cathode circuit of tube I4 by capacitor 39 and inductively coupled to the cathode circuit of tube M by chokes 3d, 32, loosely intercoupled and wound on a common core. Terminal 4% is directly connected to the anode circuit of tube [4, capacitively coupled to the cathode circuit of tube [3 by capacitor 38, and inductively coupled to the cathode circuit of tube l3 by chokes 31, 35, loosely intercoupled and wound on a second common core. The high potential terminals of cathode chokes 34, 3!, respectively, are coupled to output terminals A l, 39, respectively, by capacitors 39, 38, respectively.

In the Fig. 2 embodiment terminal 4| is coupled to the plate of the driver tube (not shown) corresponding to tube 2 (Fig. l), by resistor 25, and terminal 40 is coupled to the plate of the driver tube (not shown) corresponding to tube I (Fig. l) by resistor I9. The network 4!, 25, 2%, 27 in effect regeneratively couples the output reactor 32 to the input circuit of tube is. Similarly, the network 4! i9, 20, 2! in effect regeneratively couples the output reactor 35 to the input circuit of tube l3. These networks increase the effective drive of the driver tubes (corresponding to tubes l and 2). The driver anodes are coupled to their anode supply +13 by these networks.

"in describing the operation of the Fig. 2 circult, it will be assumed that the signal'applied to the grid of tube l 3 is going more negative and that the signal applied to the grid of tube is is going more positive. Tube [3 becomes less conductive, and its anode becomes more positive. Tube I4 becomes more conductive, and its cathode becomes more' positive. Both of these factors render terminal 4| more positive, since it is directly connected to the anode of tube l3 and capacitively coupled to the cathode of tube l4 (by 39) and inductively coupled to the cathode of tube [4 (by 34, 32).

At the same time network 25 couples back to the anode circuit of the driver tube, corresponding to tube 2, a positive going voltage, increasing the positive drive of that tube on tube M. Tube I4 becomes more conductive, and its anode less positive. Tube l3 becomes less conductive, and its cathode less positive. Both of these fee-- tors render terminal 4!] less positive, since it is directly connected to the anode of tube is and capacitively coupled to the cathode of tube [3 (by 38) and inductively coupled to the cathode of tube [3 (by 3!,

It will be seen that each output terminal is. so arranged that four events occur. In the case of terminal 4 l for example:

First, a voltage is impressed on it from the anode of tube l3;

Second, a voltage is impressed on it from the cathode of tube it, through capacitor 39;

Third, a voltage is impressed on it from the 1 cathode of tube l4, through windings 34, 32;

Fourth, it impresses on the output of the tube,

driving tube [4, such a voltage as to increase the drive of that tube (corresponding to 2, Fig. 1).

As to terminal 43:

First, a voltage is impressed on it from the anode of tube l4;

Second, a voltage is impressed on it from the cathode of tube l3, through capacitor 33;

Third, a voltage is impressed on it from the In further describing the operation of the Fig. 2 embodiment, it will now be assumed that the signal applied to the grid of tube i3 is going more positive and the signal applied to the grid of tube I4 is going more negative. Tube I3, under this assumed condition, becomes more conductive, and its anode becomes less positive. Tube I4 becomes less conductive, and its cathode becomes less positive. Both of these factors render terminal 4! less .positive, since it is directly connected to the anode of tube 13 and both capacitively and inductively coupled to the cathode of tube l4. At the same time network 25 couples back to the anode of the driver tube, corresponding to tube 2, a negative going voltage, increasing the negative drive of that tube on tube l4. Tube I4 becomes less conductive and its anode more positive. Tube l3 becomes more conductive and its cathode more positive. Both of these factors render terminal 42 more positive, since it is directly coupled to the anode of tube l4 and both capacitively and inductively coupled to the cathode of tube l3.

It will accordingly be clearly understood that the characteristics of both tubes are manifested at each output terminal. The Fig. 2 amplifier accordingly permits reasonable tube tolerances and operates very well even when the tubes l3 and 14 are not precisely matched.

Referring now specifically to the Fig. 3 embodiment, it departs from the Fig. 2 embodiment in four respects:

First, a center-tapped choke having two winding portions 3! and 34' is substituted for't'he individual chokes 3i and 34;

!3 and M, respectively. These terminals are only capacitively coupled to those cathode circuits, because there is no mutual inductive coupling between the plate chokes and the cathode chokes. The Fig. 3 embodiment represents a distinct advance in the art, in that capacitors,

6 'evenof the large size required for the Fig. 3cmbo'dimentyare far'superior to bifilar windings or other mutually coupled inductors, particularly for high power work or at high frequencies.

In the Fig. 3 embodiment coupling capacitors 36, 31 are provided at the output terminals 40 and 4|. It will be understood, of course, that such capacitors may also be employed in the Fig.

2 embodiment.

The Fig. 5 embodiment is very similar to the Fig. 3 embodiment and differs therefrom in the following respects:

First, the feedback networks, including resistors I9 and-25, are omitted, so that the driver tube anodes are directly connected to their anode current supply source;

Second, the output is taken from a secondary winding 42 which is mutually inductively reliaitegato the primary winding provided by choke The operation of the Fig. 5 embodiment issubstantially the same as that of the Fig. 3 embodiment, the output signal being taken from secondary 42. The fixed-bias source C is not shown in this embodiment.

The Fig. '7 embodiment is substantially identical to-that shown in Fig.2 and differs therefrom in the following respects:

First, a single core is provided for all of the four chokes;

Second, a center-tapped winding having portions 3| and 34' is substituted for the separate cathode chokes 3 I, 34;

Third, a single center tapped choke having portions 32' and 35' is substituted for the separate chokes 32 and 35 shown in the Fig. 2 embodiment;

Fourth, the feedback networks, including resistors I9 and 25, are omitted;

Fifth, the fixed grid bias source (-C) is omitted.

The operation of the Fig. 7 embodiment will be apparent from the detailed description of the construction and operation of Fig. 2.

Referring now to Fig. 6, this embodiment resembles that of Fig. 2 and differs therefrom in the following respects:

First, a push-pull transformer input is provided by a transformer 44 in lieu of the resistance-capacitance networks 20, 2| and 26, 21;

Second, a transformer push-pull output is provided by a transformer 58 Which has its primary connected between the anodes of the tubes l3, [4, the secondary providing the output signal;

Third, the source of fixed-bias is not shown;

Fourth, a single choke 3!, 34' is substituted for the individual cathode chokes;

Fifth, a single choke 32, 35' is substituted for the two individual plate chokes;

Sixth, the primary of the output transformer has a center tap to which the positive terminal of the space current course (+13) is connected.

The Fig. 4 embodiment substantially the same as'the Bembodiment except that the feedback networks including resistors 19 and .25 are omitted, and the details of a suitable push pull driving arrangement are shown.

InFig. 4 there is shown a driver tube H, a phase invertertube'iZ, and a pair of amplifier tubes or tricdes t3 and M, the latter. together with associated circuit arrangements, beingineluded within the rectangular outline. It will. be understood that the novel arrangements of the invention are shown within the: block diagram sofar asthe Fig. 4 embodiment is concerned,

,resistor H, by-passed by a capacitor [8.

and that any conventional network for applyin an input signal in push-pull to the amplifier shown within this block may be employed. In the, specific embodiment, a single-ended or unbalanced audio signal is applied to the control electrode E of amplifier tube I I, the latter being provided with a grid resistor i5 and a cathode The signal output of tube 1 l is applied to the control electrode of amplifier tube 53 through a conventional coupling network, comprising a load resistor l9, a coupling capacitor 20, and a grid resistor 2i. The control electrode of tube 12, which tube is provided with a cathode resistor 22, by-passed by a capacitor 23, is driven from an adjustable tap 2 1 on resistor 21. The output of tube !2 is coupled to the input of tube 54 through a conventional network comprising plate load resistor 25, coupling capacitor 26, and grid resistor 27. The input circuit arrangements supply pushpull input signals to the amplifier unit. Any of the conventional expedients may be employed for that purpose.

Amplifier tube 13 has a cathode circuit comprising a series combination of a resistor Bil and an iron core choke 3i and a plate circuit comprising an iron core choke 32. Tube is is larly provided with a cathode circuit comprising a resistor 33 and an iron core choke and an anode circuit comprising an iron core choke 35'. The junction of the cathode chokes is grounded and the junction of the plate chokes is connected to the positive terminal of a source of space current (not shown), indicated by the symbol +3. In this embodiment the output signals are taken in push-pull from the two cathodes by a pair of capacitors 35 and 31 which are individually connected to the cathodes of tubes 53 and it. It will be observed that a push-pull amplifier is provided by the elements I3, I l, 30, 3t, 32, 33, 34', 35', 35, and 3?. In accordance with the invention greater fidelity and power output is obtained by capacitively coupling the plate circuit of one tube and the cathode circuit of another to each output terminal. of tube 14 is coupled as by a capacitor to the high potential side of resistor 35. Additionally, the anode of tube 13 is coupled as by a capacitor 39 to the high potential side of resistor 33.

The operation of the Fig. l embodiment will be readily apparent in the light of the foregoing description. It will be assumed that the audio signal voltage input to tube ii is on the positive portion of its cycle, so that the plate output of tube 1 i is on the negative portion of its cycle, and the input to tube i3 is accordingly negative. Tube 12 acts as a phase inverter and inverts the output of tube ii, applying a positive signal to tube i i. Output terminal at is then on the negative portion of its signal by reason of two factors:

(1 the decreased conductivity of tube i3 which renders the high potential side of its cathode circuit more negative; (2) the increased conductivity of tube M, which renders its anode output more negative. Similarly, the output terminal 4! is in the positive portion of its cycle, the decreased conductivity of tube i3 causing its anode to become more positive and the increased conductivity of tube as causing the high potential side of its cathode circuit also to become more positive. When the signal input applied to tube H is on the negative portion of its signal, terminal to then is on the positive portion of its cycle and terminal 4! is on the negative portion of its cycle.

In one successful embodiment of the present Specifically, the anode invention according to Fig. 4, the following illustrative parameters were found to be satisfactory:

Element Description Resistor l6 1 megohm Resistor i1 500 ohms Resistors 2|, 24 1 megohm potentiometer Resistor 21 1 megohm Resistor 22 500 ohms Resistor l9 10,000 ohms Resistor 25 10,000 ohms Resistor 30 500 ohms Resistor 33 500 ohms Capacitor 38 mfds. Capacitor 39 100 mfds. Capacitor 36 100 mfds. Capacitor 3? 100 mfds. Capacitor I8 100 mfds. Capacitor 20 .25 mfd. Capacitor 23 100 mfds. Capacitor 26 .25 mfd.

50 henries center-tapped 333E: 50 henries center-tapped Tube H small triode Tube I2 small triode Tube is 2A3 Tube l4 2A3 In Fig. 4 the output is taken directly from the cathode circuits of tube It, Hi, terminal to and ll being directly connected to the cathodes. It will be understood, of course, that the output may be taken from points 59A. and MA (i. e., the anodes of the tubes).

Class A operation is preferred for all embodiments of this invention.

It will be understood that the capacitors 38 and 3% which are used in the Fig. 2 and Fig. '7 embodiments, for example, are considerably smaller in value than the capacitors 3S and 39 which are employed in the Fig. i embodiment, for which illustrative parameters have been given. It will also be understood that the term loosely coupled as used herein is intended to designate the types of coupling wherein no special precautions and designs are employed to assure tight coupling. It will further be understood that the signal output can be taken from the amplifier in various ways, such as by a transformer (Fig. 6) and capacitors (Fig. 7). It will be obvious to those skilled in the art that the output signal can also be taken by tapping down either the plate or the cathode chokes or by the provision of a tertiary winding.

While there have been shown and described what are considered to be the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various modifications and substitutions of equivalents may be made therein Without departing from the true scope of the invention as defined in the appended claims;

I claim:

1. In combination, a wave signal amplifier comprising a pair of push-pull connected amplifier tubes each having a cathode circuit, a control electrode, and an anode circuit, a common inductive reactance in said anode circuits and connected directly between the anodes, said anodes providing output terminals, means for capacitively coupling the cathode of one of said tubes to the anode of the other tube, means for capacitively coupling the cathode of said other tube to the anode of said one tube, a driving circuit comprising a second pair of push-pull connected tubes each having an anode, resistarise-capacitance coupling means for coupling the anodes of said driver tubes to the control electrodes of said amplifier tubes, a source of space current having its positive terminal connected to said reactance to provide anode voltage for the amplifier tubes, and resistors directly cross-connecting the anodes of the amplifier tubes to the anodes of the driver tubes, so that said space-current source provides anode voltage for the driver tubes.

2. The combination of two push-pull connected driver tubes each having an output anode circuit, a pair of push-pull connected output tubes each having a cathode and anode, means for A. C. coupling the driver tubes to the output tubes, an anode load connected to the anodes of both of said output tubes, a direct current power source having a positive terminal symmetrically connected to said load and a negative terminal symmetrically connected to the oathodes of said output tubes, a pair of individual means for A. C. coupling the cathode of each output tube to the anode of the other output tube, each of said individual means being in series circuit relation to said power source so as to be charged by said source, and resistors directly cross-connecting the anodes of said output tubes to the anodes of the driver tubes, so that there is supplied to the anodes of said driver tubes D. C. power together with regeneratively fedback A. C. components from said output tubes.

3. A push-pull wide band amplifier, comprising, a first electronic amplifier tube having a first anode, cathode and control electrode, a second electronic amplifier tube having a second anode, cathode and control electrode, a negative volttage terminal, a positive anode voltage terminal, conductive means including a resistor and a first reactive impedance directly connected between said negative terminal and said first cathode, conductive means including a resistor and a second reactive impedance directly connected between said negative voltage terminal and said second cathode, a third impedance directly con- 10 nected between said positive anode terminal and said second anode, a fourth impedance directly connected between said positive anode terminal and said first anode, a condenser of substantially zero impedance at the lowest frequency for which said amplifier is designed directly coupling said first cathode to said second anode, and a condenser of substantially zero impedance at the lowest frequency for which said amplifier is designed directly coupling said first anode to said second cathode, means for applying input signals in push-pull relation to said control electrodes, and means for biasing said control electrodes to provide anode current flow in at least one of said tubes in response to any finite signal, said first, second, third and fourth impedance being equal to each other for all frequencies of said Wide band.

4. The combination in accordance with claim 3 wherein said impedances are inductances.

5. The combination in accordance with claim 3 wherein said first and second impedances are inductances wound on a common core, and wherein said third and fourth impedances are inductances wound on a common core.

RONALD J. ROCKWELL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,161,844 Babler June 13, 1939 2,201,345 Percival May 21, 1940 2,202,506 Robin May 28, 1940 2,273,997 Rubin Feb. 24, 1942 2,281,345 Willwacher et a1. Apr. 28, 1942 2,284,181 Usselman May 26, 1942 2,313,330 Finch Mar. 9, 1943 2,407,074 Green Sept. 3, 1946 2,411,517 Busignies Nov. 26, 1946 2,446,025 Rockwell July 27, 1948 2,477,074 McIntosh July 26, 1949 2,502,822 Burton Apr. 4, 1950 FOREIGN PATENTS Number Country Date 892,851 France May 23, 1944 

